Addendum 1

Dr. Arthur Miller has written for us the following historical sketch concerning the development of the Sanborn 374 crystal microphone:

"Early in 1939 I was approached by either Doctor Paul Dudley White or Doctor W. Trevor Cooke about the possibility of recording pulse waves simultaneously with the electrocardiograph using the two-string galvanometer system which was then the heart of the electrocardiograph apparatus in the cardiac laboratory of the Massachusetts General Hospital.

"I realized that the pressure pulsations to be recorded, in terms of the air pressure which would reach a pick-up device such as a microphone, would be relatively enormous in comparison to the sound waves which constituted the usual input to the microphone. Since the output of a simple crystal microphone in response to sound waves is measured in millivolts, this would imply that the signal generated by the pulse waves would be measured in hundreds of millivolts.

"We had been building vacuum tube amplifier type electrocardiographs since 1935 and so we were able to think in terms of a system which had a high input impedance, sensitivity in the millivolt range, and response characteristics suitable for the frequency spectrum of human cardiac events.

"Obviously the crystal microphone could be connected to the input of one of our ECG amplifiers, but just as obviously the combination would seriously distort the pulse wave shape. The microphone itself looks like a generator in series with a small capacitance, and the amplifier input impedance which would have to be provided in the amplifier to avoid serious distortion due to the small microphone capacitance would have to be two orders of magnitude greater than we actually had in our amplifier.

"Since, however, we had millivolt sensitivity, while the microphone generated hundreds of millivolts, we had the opportunity to trade signal strength for equalization.

"The simplest equalizer circuit consisted of a large capacitor hung directly across the microphone. A value of one microfarad was large enough to provide reasonable fidelity of reproduction. The output signal reaching the amplifier was reduced in proportion to the size of this shunt capacitor, but, even for this large value of one microfarad, several millivolts of pulse wave signal remained. We adapted one of our portable battery operated cardiographs of that era for use with the shunted crystal microphone as an input, while the amplifier output drove one of the galvanometers of the two-channel ECG apparatus in the Cardiac Lab.

"About the same time a doctor working at the Boston City Hospital was looking for a device which would allow him to record the pressure pulsations of the spinal fluid when he did a spinal tap. He had observed the small pulsations at the top of the column of spinal fluid in a glass tube connected to the needle. We coupled the crystal microphone to this same glass tube (after the tap had been made) by a short hose. This trapped a small volume of air above the spinal fluid. The stiffness of this trapped air reduced the motion of the fluid in the fluid column, but, in doing so the pressure variations were transmitted more faithfully to the microphone diaphragm. For this application the pulse wave pick-up was connected to one of the standard portable Cardiettes which contained its own photographic recorder.

"Several recordings of this kind were made, but I don't think the procedure was written up, and at this time I can't recall the name of the doctor who did the work or whether he found anything of physiological significance in the tracings."